1. Download FitNesse from fitnesse.org
  2. Place the downloaded zip file in some directory (hint, this next step creates a directory called FitNesse under where you execute this command). For reference, I'll be using ~/src/. The next step will create ~/src/FitNesse
  3. Execute the jar file once to get it extracted:
java -jar fitnesse.jar
Note, this also attempts to start FitNesse on port 80. After this is done, you'll either see an error message (if port 80 is in use):

FitNesse cannot be started...
Port 80 is already in use.
Use the -p <port#> command line argument to use a different port.[[code]]
Or you'll see a message similar to this:
        port:              80
        root page:         fitnesse.wiki.FileSystemPage at ./FitNesseRoot
        logger:            none
        authenticator:     fitnesse.authentication.PromiscuousAuthenticator
        html page factory: fitnesse.html.HtmlPageFactory
        page version expiration set to 14 days.

If you see the second option, kill FitNesse for now (hit ctrl-C).